Alkylation of aromatic compounds with a C2 to C4 olefin and transalkylation of polyalkylaromatic compounds are two common reactions for producing monoalkylated aromatic compounds. Examples of these two reactions that are practiced industrially to produce ethylbenzene are the alkylation of benzene with ethylene and the transalkylation of benzene and diethylbenzene.
Combining alkylation and transalkylation can thus maximize ethylbenzene production. Such a combination can be carried out in a process having two reaction zones, one for alkylation and the other for transalkylation, or in a process having a single reaction zone in which alkylation and transalkylation both occur.
A key operating variable directly related to operating efficiency of alkylation process is the molar ratio of aryl groups per alkyl group. The lower the ratios, the lower the amounts of benzene required to recover/recycle, the lower the capital and utility cost would be. The numerator of this ratio is the number of moles of aryl groups passing through the reaction zone during a specified period of time. The number of moles of aryl groups is the sum of all aryl groups, regardless of the compound in which the aryl group happens to be. In the context of ethylbenzene production, for example, one mole of benzene, one mole of ethylbenzene, and one mole of diethylbenzene each contribute one mole of aryl group to the sum of aryl groups. The denominator of this ratio is the number of moles of alkyl groups that have the same number of carbon atoms as that of the alkyl group on the desired monoalkylated aromatic and which pass through the reaction zone during the same specified period of time. The number of moles of alkyl groups is the sum of all alkyl and alkenyl groups with the same number of carbon atoms as that of the alkyl group on the desired monoalkylated aromatic, regardless of the compound in which the alkyl or alkenyl group happens to be, except that paraffins are not included. In the context of ethylbenzene production, the number of moles of ethyl groups is the sum of all ethyl and ethenyl groups, regardless of the compound in which the ethyl or ethenyl group happens to be, except that paraffins, such as ethane, propane, n-butane, isobutane, pentanes, and higher paraffins are excluded from the computation of the number of moles of ethyl groups. For example, one mole of ethylene and one mole of ethylbenzene each contribute one mole of ethyl group to the sum of ethyl groups, whereas one mole of diethylbenzene contributes two moles of ethyl groups and one mole of triethylbenzene contributes three moles of ethyl groups. Butylbenzene and octylbenzene contribute no moles of ethyl groups
Advancements in zeolites and catalysts have enabled the aromatic alkylation process to operate at lower aryl to alkyl ratios. The catalysts typically include a relatively high content of zeolite in order to ensure good activity, activity stability and stable long-term operation. Currently, aromatic alkylation catalysts including UZM-8 zeolite have a zeolite content greater than 50 wt %.
Zeolite is synthesized using organic templates, which are removed via calcinations in the catalyst preparation. Because of the heat and steam evolved during the calcination, the zeolite would incur appreciable structural and framework damages. The degree of damage is related to the degree of hydrothermal severity, which is proportional to the amount the zeolite in the catalyst. Furthermore, at high zeolite contents, the zeolite in the catalyst tends to agglomerate, reducing the effective utilization of zeolite. Lastly because of the high cost of zeolites, catalysts containing high levels of zeolites and processes using those catalysts are also expensive.